Anticorrosion treatment solution and uses

ABSTRACT

The present invention relates to a solution that is free of chromium in all its oxidising states, comprising: - at least one chemical oxidising compound, - at least one aluminium complexing agent, - at least one corrosion inhibiting compound, and -optionally, a chemical sealant compound, the solution having a pH in the range from 1 to 5. The present invention also relates to a method for treating a metal surface, comprising the application, on the surface, of a solution as defined above. The present invention additionally relates to a coating of a metal surface that can be obtained by the method for treating a metal surface as defined above, to a metal surface comprising the coating and to the use of the solution in an anti-corrosion treatment of a metal surface.

TECHNICAL FIELD

The present invention relates to a solution free of chromium in all its oxidation states, to a process for treating a metal surface, comprising the application of the solution to this surface, and also to a coating for a metal surface which may be obtained via the treatment process.

In the description hereinbelow, the references in square brackets ([ ]) refer to the list of references presented at the end of the text.

PRIOR ART

Reducing the fuel consumption of aircraft is a major challenge both for equipment manufacturers and aviators and for engine manufacturers.

In the race for the development of emerging materials, novel lighter aluminum-based alloys have been developed.

However, aluminum and its alloys are sensitive to corrosion. As a result, articles based on aluminum or its alloys must be protected against attack from the external environment which may be reflected by corrosion. Protective coatings must thus be applied thereto to protect the aluminum.

The majority of the current corrosion protection processes use compositions based on hexavalent chromium. Such compositions comprise, for example, chromium trioxide (CrO₃), potassium dichromate K₂Cr₂O₇, sodium dichromate Na₂CrO₇ or strontium chromate SrCrO₄. However, hexavalent chromium is listed among the hazardous substances prohibited by the REACH (Registration, Evaluation, Authorization and restriction of CHemicals) regulation, which is directed toward protecting human health and the environment against the hazards associated with chemical substances, while at the same time promoting the competitiveness of the chemical industry of the European Union. Its use has been totally prohibited since 2017.

Various treatments for protecting aluminum alloys against corrosion exist and are dependent on the composition of the alloy: electrochemical deposition, anodization, chemical conversion, gas-phase deposition, sol-gel coating or laser deposition coating. These various treatments are used on an industrial scale with an observed preference for the anodization and chemical conversion processes. Notably, surface treatment by chemical conversion offers several advantages such as its low cost, its ease of use and the properties of the protective layer obtained which are compliant with aeronautical specification requirements, for example. Specifically, in the aeronautical sector, anticorrosion-treated metallic parts must have properties in terms of adherence, coloring, conduction and attachment for a subsequent paint or varnish. Specifically, a paint or varnish is generally applied after the conversion layer to improve the corrosion protection. The coloring makes it possible to perform a visual control of the quality of the anticorrosion coating and is a usual feature in the specification requirements of certain clients. Moreover, adherence between the coating and the metal surface is necessary.

Several types of chemical conversion treatments using solutions free of hexavalent chromium exist and are currently marketed. Generally, these are solutions based on trivalent chromium, also denoted as Cr(III) (J.T. Qi et al.: “Trivalent chromium conversion coating formation on aluminium, Surface and Coatings Technology”, 280 (2015) 317-329 ([1]); W.-K. Chen et al.: “The effect of chromic sulfate concentration and immersion time on the structures and anticorrosive performance of the Cr(III) conversion coatings on aluminum alloys”, Applied Surface Science, 256 (2010) 4924-4929 ([2])), such as SURTEC® 650 sold by the company SURTEC, or the solution Lanthane 613.3 sold by the company COVENTYA or TCS sold by the company SOCOMORE.

Another alternative to the use of a composition based on hexavalent chromium consists in using solutions based on phosphate compounds (F. Andreatta et al.: “Addition of phosphates or copper nitrate in a fluotitanate conversion coating containing a silane coupling agent for aluminium alloy AA6014”, Progress in Organic Coatings, 77 (2014) 2107-2115 ([3])), of titanium and zirconium compounds in fluorinated medium (P. Santa Coloma et al.: “Chromium-free conversion coatings based on inorganic salts (Zr/Ti/Mn/Mo) for aluminum alloys used in aircraft applications”, Applied Surface Science, 345 (2015) 24-35 ([6]); P.D. Deck et al.: “Investigation of fluoacid based conversion coatings on aluminum, Progress in Organic Coatings”, 34 (1998) 39-48 ([9]); H. Nordlien et al.: “Formation of a zirconium-titanium based conversion layer on AA 6060 aluminium”, Surface and Coatings Technology, 153 (2002) 72-78 ([10])), of molybdenum or manganese ([6]), or of rare-earth metals such as cerium (B. Valdez et al.: “Cerium-based conversion coatings to improve the corrosion resistance of aluminium alloy 6061-T6”, Corrosion Science, 87 (2014) 141-149 ([4]); P. Campestrini et al.: “Formation of a cerium-based conversion coating on AA2024: relationship with the microstructure”, Surface and Coatings Technology, 176 (2004) 365-381 ([8])), or zirconium in fluorinated medium (H.R. Asemani et al.: “Effect of zirconium conversion coating: Adhesion and anti-corrosion properties of epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigment on carbon mild steel”, Progress in Organic Coatings, 94 (2016) 18-27 ([5]); F.O. George et al.: “Formation of zirconium-based conversion coatings on aluminium and Al— Cu alloys”, Corrosion Science, 65 (2012) 231-237 ([7])).

The advantages and drawbacks of the solutions mentioned previously are indicated in table 1. These advantages and drawbacks take into account the fact that they do not use hexavalent chromium.

TABLE 1 VARIOUS TYPES OF COATINGS Name advantages drawbacks Chemical conversion - Good system for attachment of paint/varnish, - Poor corrosion resistance, - More profitable process, - Need to apply a varnish or a paint, - Suitable for complex parts, - Fragile deposit - Conductive layer, - No size constraints Anodization - Very good corrosion resistance, - Depends on the type of alloy, - Uniform deposit - Embrittlement of the surface of the part, - Size constraints, - Long treatment times, - Limitations for complex parts Sol-gel coatings - Good corrosion resistance, - Non-uniform deposit, - Complex process - Size constraints, - Instability of the product, - Short service life of the baths/waste problems, - Can only be applied to parts of complex geometry

The current solutions do not make it possible to satisfactorily replace compositions based on hexavalent chromium. Specifically, these solutions are not as efficient in terms of corrosion resistance. Moreover, for parts of complex geometry, i.e. parts comprising recesses and/or internal areas that are not accessible such as internal pipes or tanks, or for parts which have size constraints, these solutions entail a variation of the dimensions or sides of more than 1 or 2 microns. Such a variation is unacceptable in fields such as the aeronautics field.

The solutions used for chemical conversion using solutions based on chromium(III) enable sufficient adherence of the coatings. However, the corrosion protection obtained by means of these solutions does not meet the specification requirements notably of aeronautics. Moreover, these solutions based on chromium(III) do not have corrosion resistance similar to that of solutions based on hexavalent chromium. The corrosion resistance may be evaluated by means of a test which consists in exposing a specimen of standardized size to a salt spray for a duration of 168 hours. The result of this test for a composition based on hexavalent chromium such as Alodine 1200 is of the order of less than 2.5 pits on the specimen. When this test is performed with SURTEC® 650, the results vary according to the aluminum alloy range treated. For example, for aluminum alloys of the 5000 and 6000 series, a saline spray resistance of greater than 168 hours is obtained and the corrosion resistance is thus satisfactory. For aluminum alloys of the 2000 series, satisfactory protection by applying SURTEC® 650 is obtained for a shorter period, or, in other words, the saline spray resistance is less than 168 hours. For aluminum alloys of the 7000 series, the corrosion protection is not viable (saline spray resistance of less than 100 hours) (C. Jambon: Light Metal Surface Finishing, Traitement des alliages legers, A3TS, December 3-4, 2013, Le Bourget, France ([11]); P. Frou, Etat des travaux du GIFAS pour accompagner la filière des Traitements de Surface, face aux menaces notamment du fait de REACH dans un contexte d′augmentation de cadences aeronautiques [State of the GIFAS studies to accompany the Surface Treatment channel, in the face of the threats notably arising from REACH in a context of increasing aeronautical production rates], Journee traitement de surface du pôle Aerospace Valley/DAS AMP [Surface Treatment Day of the Aerospace Valley Center/DAS AMP], Mar. 18, 2016, Toulouse, France ([12]).

Only one solution based on permanganate ion was listed, but not identified ([11]). In addition, although there are solutions for treating objects based on magnesium or magnesium alloy, it should be noted that these solutions are not suitable for the treatment of objects made of aluminum alloy or aluminum.

There is thus a real need for anticorrosion coatings free of hexavalent chromium, which overcome these defects, drawbacks and obstacles of the prior art.

DESCRIPTION OF THE INVENTION

After extensive research, the Applicant has developed a chromium-free anticorrosion protection, notably in the form of a bath for treating aluminum alloys, and also an associated treatment process.

The invention thus relates to a chromium-free chemical conversion solution which advantageously has good properties, namely:

-   good corrosion resistance, notably a variation of the corrosion     potential of the protected alloy relative to the crude aluminum     alloy (ΔE) equal to +0.3 V, a ΔE = +0.45 V relative to the coating     obtained using Alodine 1200, an annual corrosion of less than 5     µm/year whereas protection with an Alodine 1200 bath limits the     annual corrosion to only about 15 µm/year, -   good attachment of varnish and/or paint, -   absence of size constraints: production of a layer of 1-2 or even     less than 1 µm in thickness, -   production of a colored and conductive deposit: orange-yellow color, -   self-repair property.

Treatment of a part made of aluminum or of aluminum alloy with the solution of the invention advantageously allows:

-   oxidation of the surface of the aluminum alloy, enabling the     formation of a thick protective layer, -   formation of a colored passivating layer which may comprise     manganese oxides (MnO₂(s), MnO(s)), -   treatment of the porosity problems encountered, for example, with a     layer of manganese dioxide MnO2(s) alone, -   limitation of the aluminum concentration in the deposit by means of     the action of aluminum-complexing agent(s), -   the incorporation of corrosion inhibitors, improving the corrosion     resistance, -   the trapping of potassium permanganate, giving the deposit     self-regenerating properties.

The invention is of interest for the aeronautical sector (civil and military), which is impacted by the REACH regulation, notably equipment manufacturers, aviators and engine manufacturers, and also for the sectors which use chemical conversion with chromium(VI), such as the motor vehicle industry, the building sector and street furniture.

Thus, a first subject of the invention relates to a solution free of chromium in all its oxidation states, comprising:

-   at least one oxidizing chemical compound, -   at least one aluminum-complexing agent, -   at least one corrosion-inhibiting compound, and -   optionally a plugging chemical compound,

said solution having a pH ranging from 1 to 5.

For the purposes of the present invention, the term “solution” means a composition in liquid form, in which parts comprising aluminum or an aluminum alloy may be soaked. The solution of the invention is advantageously a chemical conversion solution, i.e. it is suitable for use in the context of a chemical conversion treatment, or for enabling the chemical conversion of aluminum and alloys thereof.

For the purposes of the present invention, the term “aluminum alloy” means an alloy of which the main constituent is aluminum. The alloy may also comprise at least one other component chosen from copper, silicon, magnesium, titanium and zinc. The at least one other component may be present in the alloy in a mass percentage of between about 0.10% to about 21.00% by weight relative to the weight of the alloy. The aluminum alloy may be, for example, an alloy of the 2000 series of the aluminum alloy classification (Aluminium Association, Washington DC 2006, United States), for instance the alloy 2024 or 2618, of the 6000 series or of the 7000 series, for instance the alloy 7075 or 7175.

The solution of the invention may “comprise” or “consist of” the elements indicated previously. In the case where it “comprises” the indicated elements, it may then include these elements and also other elements, with the exception of chromium. In the case where it “consists of” these elements, it then solely includes the elements listed previously, to the exclusion of any other element.

For the purposes of the present invention, the term “free of chromium in all its oxidation states” means a total absence of chromium in the solution of the invention. In other words, there is a total absence of chromium in its observable oxidation states ranging from -II to VI, and notably hexavalent chromium, in the solution of the invention. The chromium ion may notably be chromate or dichromate. The absence of chromium may be due to the fact that no element containing chromium, notably hexavalent chromium, is added during the process for preparing the solution of the invention.

The pH of the solution may be from 1.0 to 5.0, the limits being included. It may be, for example, from 1.2 to 4.8, or from 1.5 to 4.5, or from 2.0 to 5.0, or from 2.2 to 4.8, or from 2.5 to 4.5, or from 3.0 to 4.0, or from 3.2 to 3.8, the limits being included. In one embodiment, if the pH of the solution fluctuates beyond these values, it is possible to bring the pH back within the indicated values, for example by adding a strong acid such as sulfuric acid (H₂SO₄) to lower the pH or a strong base such as potassium hydroxide (KOH) to increase the pH. Advantageously, a pH in the region of 4.0 may make it possible to obtain the best corrosion protection performance, notably a ΔE = +0.3 V relative to the crude alloy, i.e. the alloy without coating due to the solution of the invention.

For the purposes of the present invention, the term “oxidizing chemical compound” means any chemical compound that is capable of receiving at least one electron from another chemical species during a redox reaction. The oxidizing chemical compound of the solution of the invention may be chosen from the group comprising permanganate salts, molybdate salts, persulfate salts and hydrogen peroxide, and mixtures thereof. Advantageously, the concentration of the oxidizing chemical compound in the solution may be between 0.01 and 0.45 mol/L, the limits being included, for example from 0.05 to 0.40 or from 0.1 to 0.4 or from 0.2 to 0.3 mol/L, the limits being included.

In the case of permanganate or molybdate salts, the permanganate or molybdate ions contained in the solution of the invention may be associated with any suitable type of counterion, for example potassium permanganate KMnO₄ or sodium permanganate NaMnO₄ and sodium molybdate Na₂MoO₄, potassium molybdate K₂MoO₄ or ammonium molybdate (NH₄)₂MoO₄. Advantageously, the permanganate ion is used since it is a source of manganese and the molybdate ion is used as source of molybdenum.

In the case of the persulfate salts, the persulfate ion may be peroxomonosulfate SO₅ ²⁻ or peroxodisulfate S₂O₈ ²⁻. The persulfate salt may be chosen, for example, from all the known persulfate salts, for example from ammonium persulfate, sodium persulfate, potassium persulfate, potassium hydrogen persulfate and the triple salt of potassium monopersulfate.

For the purposes of the present invention, the term “complexing agent” means any compound which makes it possible to react with a metal, notably aluminum and alloys thereof, and thus to form a soluble complex compound. Advantageously, the aluminum-complexing agent also acts as a corrosion inhibitor. In this respect, it can advantageously make it possible to prevent or limit corrosion on a metallic part, notably aluminum and alloys thereof, with the exception of the chromium ion. The corrosion may be evaluated by measuring the number of pits on the surface of the metallic part, in a given time and under given conditions. The properties required are an absence of pits after 168 hours of exposure to a salt spray test according to the standard ASTM B117.

The aluminum-complexing agent contained in the solution of the invention may be a fluorinated salt or a mixture of fluorinated salts, an organic compound chosen from gluconates, citrates, oxalates, acetates and formates, or any mixture thereof. The fluorinated salt may be chosen, for example, from hexafluorozirconates, hexafluorotitanates, hexafluorosilicates and any mixture thereof. Among the gluconates, it may be, for example, sodium gluconate, potassium gluconate, calcium gluconate or ammonium gluconate. Among the citrates, it may be, for example, sodium citrate, potassium citrate or ammonium citrate. Among the oxalates, it may be sodium oxalate, potassium oxalate or ammonium oxalate. Among the acetates, it may be sodium acetate, potassium acetate or ammonium acetate. Among the formates, it may be sodium formate, potassium formate or ammonium formate.

For the purposes of the present invention, the term “corrosion-inhibiting compound” means any compound that is capable of reducing the rate of corrosion of a metal surface under the usual conditions of use. The corrosion-inhibiting compound may be chosen from rare-earth metal, tungstate, vanadate, phosphate and cerium(III) salts, zirconium, titanium or silicon salts. These compounds may be introduced in a minor dose, for example from 0.1% to 5% by mass, notably from 0.5% to 4.0% by mass or from 1.0% to 3.0% by mass. Only one inhibitor or a mixture of inhibitors may be used to improve the corrosion resistance of the coating.

For the purposes of the present invention, the term “plugging agent” means any compound which makes it possible to prevent the presence of porosities or of thickness heterogeneity of the deposited layer of precipitate. The plugging agent that may be contained in the solution of the invention may be a compound based on phosphate, phosphonate, polyphosphate or iron ions. In this respect, the phosphate ion may be associated with any suitable type of counterion. It may be, for example, potassium or sodium hydrogen phosphates KH₂PO₄, K₂HPO₄, NaH₂PO₄ or Na₂HPO₄ or phosphoric acid H₃PO₄. Advantageously, the phosphate ion may be used as plugging agent, i.e. it has the function of unifying the thickness and the chemical composition profile of the manganese and aluminum oxide layer formed so as to make it more passivating. The plugging agent may also be an iron salt of the type such as iron sulfate Fe₂(SO₄)₃, ferric chloride FeCl₃, potassium ferricyanide (K₃Fe(CN)₆) or iron gluconate or oxalate. In the solution of the invention, the concentration of plugging agent may be, for example, between 0.001 and 0.20 mol/L, the limits being included, notably from 0.010 to 0.18 mol/L, or from 0.050 to 0.18 mol/L, or from 0.08 to 0.18 mol/L or from 0.10 to 0.15 mol/L, the limits being included.

For example, the chemical conversion solution of the invention may be a solution in which:

-   the oxidizing chemical compound is potassium permanganate, -   the plugging agent, when it is present, is a compound chosen from     potassium hydrogen phosphate, phosphoric acid or an iron salt, and -   the complexing agent is a mixture of hexafluorozirconic acid,     hexafluorotitanic acid and hexafluorosilicic acid.

In the solution of the invention, the concentration of permanganate ion may be between 0.01 and 0.45 mol/L, the limits being included. The concentration may be, for example, from 0.05 to 0.40, or from 0.1 to 0.4 or from 0.2 to 0.3 mol/L, the limits being included.

In the solution of the invention, the concentration of phosphate ions may be between 0.001 and 0.20 mol/L, the limits being included. The concentration may be, for example, from 0.010 to 0.18 mol/L, or from 0.050 to 0.18 mol/L, or from 0.08 to 0.18 mol/L or from 0.10 to 0.15 mol/L, the limits being included.

In the solution of the invention, the concentration of complexing agent may be between 0.001 and 0.15 mol/l, the limits being included. The concentration may be, for example, from 0.005 to 0.15 mol/L, or from 0.010 to 0.15 mol/L, or from 0.05 to 0.15 mol/L or from 0.08 to 0.12 mol/L, the limits being included.

Another subject of the invention relates to a process for treating or coating a metal surface, comprising the application to said surface of a solution as defined previously. The treatment may be, for example, an anticorrosion treatment.

The process may also comprise at least one step of pretreating the surface. Thus, the process of the invention may be composed of only one or a succession of pretreatment steps, followed by a step of treating with the solution of the invention.

The pretreatment step may be of the type (1), (2) or (3) below, and may successively comprise the following steps:

-   (1) alkaline degreasing of the surface, followed by nitric pickling     of the surface and then hydrofluoric pickling of the surface, -   (2) alkaline degreasing of the surface and ferric sulfo-nitro     pickling of the surface, or -   (3) alkaline degreasing of the surface, followed by basic soda     pickling of the surface under an ultrasonic field or basic soda     pickling of the surface without an ultrasonic field.

Each type of pretreatment (1), (2) or (3) may comprise or consist of immersion in a bath maintained at a fixed temperature and for a given time followed by two rinses in cascade with demineralized water.

The treatment step may comprise or consist of immersion in a bath comprising or consisting of the solution of the invention, maintained at a fixed temperature and for a given time followed by two rinses in cascade with demineralized water.

The concentrations of the various species during the preparation of the conversion bath may be as defined above in the context of the definition of the solution of the invention. For example, the ion concentrations may be as follows:

-   the concentration of MnO₄ ⁻ ions may be equal to 0.01-0.45 mol/L, -   the concentration of H₂PO₄ ⁻ ions may be equal to 0.05-0.2 mol/L, -   the concentration of H₂ZrF₆ ions may be equal to 0.005-0.1 mol/L, -   the concentration of Ce(III) ions may be equal to 0.003-0.3 mol/L,

which may correspond, for example, to masses initially introduced of about:

-   1.5 to 75 g/L of potassium permanganate KMnO₄, -   5 to 30 g/L of potassium hydrogen phosphate KH₂PO₄, -   1.4 to 27 ml/L of 50% hexafluorozirconic acid, -   1 to 10 g/L of cerium(III) nitrate.

In the context of the process of the invention, the object to be treated may have a metal surface made of aluminum or of aluminum alloy.

Advantageously, the process of the invention may make it possible to produce a coating on a metal surface.

Thus, another subject of the invention relates to a coating for a metal surface that may be obtained via the process for treating a surface as defined previously. Advantageously, the coating of the invention may be a compact layer, which has a thickness of less than 1 µm and which is adherent, for the application of a varnish or a paint. Advantageously, other features of this coating are possibly totally or partially the following:

-   have a coloring that is visible, uniform and free of defects, -   be continuous, uniform, adherent, without any discontinuities     (cracks, holes, etc.), without dusting, smooth and identifiable     (i.e. colored or easily detectable), -   have perfect heat resistance up to 80° C. and not deteriorate, -   withstand a salt spray for at least 168 hours with less than 1.5     pits/dm² and no pits with a diameter of greater than 0.8 mm after     168 hours, -   have a layer weight of between 0.42 g/m² and 1.2 g/m², -   have good dry and wet adherence for any corrosion primer, -   be insoluble in alcohols, water and solvents, but be soluble in     alkaline products and strong acids, -   have an electrical continuity of less than 5000 µohms/inch² in the     initial state and not exceed 10000 µohms/cm² after 168 hours of     exposure to a salt spray.

Another subject of the invention relates to a metal surface, notably made of aluminum or of aluminum alloy, comprising a coating as defined previously.

Another subject of the invention relates to the use of a solution as defined previously, for treating a metal surface, notably made of aluminum or of aluminum alloy.

The treatment may be chosen from:

-   an anticorrosion treatment (preventive), -   a pretreatment for an application of paint, -   a local repair of a coating of parts already treated, -   pickling of parts which have undergone anodization.

Other advantages may also appear to a person skilled in the art on reading the examples below.

EXAMPLES OR EMBODIMENTS Example 1: Preparation of a Chemical Conversion Solution of the Invention

The production of a chemical conversion solution consists in dissolving in water several potassium permanganate, potassium hydrogen phosphate, cerium nitrate and hexafluorozirconic acid salts in the following proportions:

-   1.5 to 75 g/L of potassium permanganate KMnO₄, -   5 to 30 g/L of potassium hydrogen phosphate KH₂PO₄, -   1.4 to 27 ml/L of 50% hexafluorozirconic acid, -   1 to 10 g/L of cerium(III) nitrate.

The preparation is performed at 60° C. with a dissolution time for all the salts of about 1 hour.

Example 2: Treatment of a Metal Surface Made of Aluminum or Alloy Using The Chemical Conversion Solution of the Invention

The protocol for treating a part made of aluminum or aluminum alloy is composed of several steps:

-   immersion of the part for few minutes (2-6 minutes) in an alkaline     degreasing bath which is chosen from the various solutions existing     in a surface treatment workshop, -   rinsing in a rack plating bath and then in a bath of demineralized     water, -   immersion of the part for a few minutes (2-6 minutes) in an acidic     pickling bath of composition available in surface treatment (ST)     workshops, -   rinsing in a rack plating bath and then in a bath of demineralized     water, -   immersion in the chemical conversion bath which is the subject of     the present invention for a time of between 2 and 10 minutes     depending on the alloy to be treated, -   rinsing in a rack plating bath and then in a bath of demineralized     water.

LIST OF REFERENCES

1. J.T. Qi et al.: “Trivalent chromium conversion coating formation on aluminium, Surface and Coatings Technology”, 280 (2015) 317-329.

2. W.-K. Chen et al.: “The effect of chromic sulfate concentration and immersion time on the structures and anticorrosive performance of the Cr(III) conversion coatings on aluminum alloys”, Applied Surface Science, 256 (2010) 4924-4929.

3. F. Andreatta et al.: “Addition of phosphates or copper nitrate in a fluotitanate conversion coating containing a silane coupling agent for aluminium alloy AA6014”, Progress in Organic Coatings, 77 (2014) 2107-2115.

4. B. Valdez et al.: “Cerium-based conversion coatings to improve the corrosion resistance of aluminium alloy 6061-T6”, Corrosion Science, 87 (2014) 141-149.

5. H.R. Asemani et al.: “Effect of zirconium conversion coating: Adhesion and anti-corrosion properties of epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigment on carbon mild steel”, Progress in Organic Coatings, 94 (2016) 18-27.

6. P. Santa Coloma et al.: “Chromium-free conversion coatings based on inorganic salts (Zr/Ti/Mn/Mo) for aluminum alloys used in aircraft applications”, Applied Surface Science, 345 (2015) 24-35.

7. F.O. George et al.: “Formation of zirconium-based conversion coatings on aluminium and Al—Cu alloys”, Corrosion Science, 65 (2012) 231-237.

8. P. Campestrini et al.: “Formation of a cerium-based conversion coating on AA2024: relationship with the microstructure, Surface and Coatings Technology”, 176 (2004) 365-381.

9. P.D. Deck et al.: “Investigation of fluoacid based conversion coatings on aluminum, Progress in Organic Coatings”, 34 (1998) 39-48.

10. H. Nordlien et al.: “Formation of a zirconium-titanium based conversion layer on AA 6060 aluminium”, Surface and Coatings Technology, 153 (2002) 72-78.

11. C. Jambon: Light Metal Surface Finishing, Traitement des alliages legers, A3TS, December 3-4, 2013, Le Bourget, France.

12. P. Frou, Etat des travaux du GIFAS pour accompagner la filière des Traitements de Surface, face aux menaces notamment du fait de REACH dans un contexte d′augmentation de cadences aeronautiques [State of the GIFAS studies to accompany the Surface Treatment channel, in the face of the threats notably arising from REACH in a context of increasing aeronautical production rates] , Journée TS du pôle Aerospace Valley/DAS AMP [Surface Treatment Day of the Aerospace Valley Center/DAS AMP, Mar. 18, 2016, Toulouse, France. 

1. A solution free of chromium in all its oxidation states, comprising at least one oxidizing chemical compound, at least one aluminum-complexing agent, at least one corrosion-inhibiting compound, and optionally a plugging chemical compound, said solution having a pH ranging from 1 to
 5. 2. The solution as claimed in claim 1, in which the oxidizing chemical compound is chosen from the group comprising permanganate salts, molybdate salts, persulfate salts and hydrogen peroxide.
 3. The solution as claimed in claim 1, in which the complexing agent is chosen from fluorinated salts and mixtures thereof, organic compounds chosen from the group comprising gluconates, citrates, oxalates, acetates and formates and a mixture of at least one fluorinated salt and of at least one of said organic compounds.
 4. The solution as claimed in claim 3, in which the complexing agent is a fluorinated salt chosen from hexafluorozirconates, hexafluorotitanates, hexafluorosilicates and a mixture thereof.
 5. The solution as claimed in any one of the preceding claims, in which the corrosion inhibitor is chosen from rare-earth metal, tungstate, vanadate, phosphate and cerium(III) salts, zirconium, titanium or silicon salts.
 6. The solution as claimed in any one of the preceding claims, also comprising a plugging chemical compound based on phosphate ions, phosphonate ions or polyphosphate ions or iron ions.
 7. The solution as claimed in claim 6, in which: the concentration of permanganate ion is between 0.01 and 0.45 mol/L, the concentration of phosphate ion is between 0.001 and 0.20 mol/l, and the concentration of complexing agent is between 0.001 and 0.15 mol/L.
 8. The solution as claimed in claim 6, in which: the chemical compound comprising permanganate ions is potassium permanganate, the chemical compound comprising phosphate ions is chosen from potassium hydrogen phosphate, phosphoric acid and an iron salt, and the complexing agent is a mixture of hexafluorozirconic acid, hexafluorotitanic acid and hexafluorosilicic acid.
 9. A process for treating a metal surface, comprising the application to said surface of a solution as defined in any one of claims 1 to
 8. 10. The process as claimed in claim 9, also comprising a step of pretreating said metal surface.
 11. The process as claimed in claim 10, in which said pretreatment step successively comprises the following steps alkaline degreasing of the surface, nitric pickling of said surface and hydrofluoric pickling of said surface, or alkaline degreasing of said surface and ferric sulfo-nitro pickling of said surface, or alkaline degreasing of said surface and basic soda pickling of said surface optionally under an ultrasonic field.
 12. The process as claimed in any one of claims 9 to 11, in which said metal surface consists of aluminum or is an aluminum alloy.
 13. A coating of a metal surface that may be obtained via the process for treating a metal surface as defined in any one of claims 9 to
 11. 14. A metal surface comprising a coating as defined in claim
 13. 15. The use of a solution as defined in any one of claims 1 to 8, in an anticorrosion treatment of a metal surface. 